They may have Lyft or Uber there, but I haven’t heard of anyone using them on the Camino, and googling around doesn’t yield much. All I’ve heard of are people taking taxis, which are everywhere. However, I couldn’t find any taxi companies that will take you all the way from the Santiago airport to Castroverde or O Cadavo, but Rome2rio lists several companies that will take you from the airport to Lugo for $130 or more. I also found another, suntransfer.com, which you can book on-line and charges 146 euros (approx. $170) to go to Lugo Maybe if you e-mail these companies directly, you can get a quote for all the way to Castroverde or O Cadavo.
Frankly, it mght be less stressful (and certainly a lot cheaper) just to hop on the airport bus to Lugo, and then pick up a taxi there for the short remaining journey, if you don’t want to stress the connection to another bus. There will be plenty of taxis at the Lugo bus station. Rome2rio shows a second bus company, beyond Empresa, which also goes from the airport to Lugo, so there should be no shortage of buses.
People at the airport will be very helpful if / when you have questions. They’re very used to befuddled pilgrims.
Breathe deep! This will work out.